Opto-electric devices such as light-emitting diodes are typically manufactured by making multiple identical copies of a device in parallel on a substrate. Substrates typically used to manufacture opto-electric devices include wafers of sapphire, gallium arsenide, indium phosphide, silicon, germanium, diamond, or ceramics. These substrates typically need to be singulated into individual devices. Singulation can be performed by first scribing the substrate with a diamond saw or laser. Scribing is defined as creating a modified region on the surface or within the volume of a substrate which facilitates cracking and thereby the separation of the substrate proximate to the scribe. U.S. Pat. No. 6,580,054 SCRIBING SAPPHIRE SUBSTRATES WITH A SOLID STATE UV LASER, inventors Kuo-Ching Liu, Pei Hsien Fang, Dan Dere, Jenn Liu, Jih-Chuang Huang, Antonio Lucero, Scott Pinkham, Steven Oltrogge, and Duane Middlebusher, issued Jun. 17, 2003 describes the basic process of scribing substrates to prepare them for singulation by cleaving, wherein scribed substrates are subjected to localized stress at the scribe location to create cracks in the substrate which propagate through the thickness of the substrate thereby separating the wafer along the scribe. Substrates are often adhesively attached to a die attach film or plastic sheet attached to a surrounding frame called a tape frame during singulation to keep the singulated devices in place in spite of being separated. Laser scribing is distinguished from laser dicing where the substrate is completely or nearly through cut with the laser or saw to perform singulation without the cleaving step.
Problems with laser scribing include selecting laser parameters to process materials with widely varying laser energy absorption characteristics, avoiding damage to substrates and devices and maintaining acceptable system throughput. Opto-electric devices are particularly difficult to laser scribe or dice because the performance of the device can be dependent upon the manner in which the device was singulated. In particular, some opto-electric devices are constructed with a backside coating which reflects stray light back out of the device thereby increasing the device output efficiency as measured in candelas per ampere. The article “Nitride-Based LEDs With a Hybrid Al Mirror+Tio2/Slo2 DBR Backside Reflector,” authors S. J. Chang, C. F. Shen, M. H. Hsieh, C. T. Kuo, T. K. Ko, W. S. Chen and S. C. Shei, describes the construction of these types of opto-electric devices. Laser scribing techniques which cause an undesirable heat affected zone adjacent to the scribes can reduce device output efficiency, as can damage which causes the reflective coating to “lift-off” or separate from the substrate. U.S. Pat. No. 7,804,043 METHOD AND APPARATUS FOR DICING OF THIN AND ULTRA THIN SEMICONDUCTOR WAFER USING ULTRAFAST PULSE LASER, inventor Tan Deshi, issued Sep. 28, 2010, describes using ultrafast lasers to scribe wafers but does not consider the need to remove coatings such as a DBR. U.S. Pat. No. 6,992,026, LASER PROCESSING METHOD AND LASER PROCESSING APPARATUS, inventors Fumitsugu Fukuyo, Keshi Fukumitsu, Maoki Uchiyama and Toshimitsu Wakuda, issued Jan. 31, 2006 discusses damage to substrates caused by pulsed lasers while scribing. This patent teaches using laser beams focused deep within the substrate to create altered zones unconnected to either the frontside or backsides of the wafer to permit cleaving while preventing damage to the substrate. U.S. Pat. No. 7,494,900, BACKSIDE WAFER DICING, inventors Richard S. Harris and Ho W. Lo, issued Feb. 24, 2009, discusses the advantages of performing backside processing of substrates but does not discuss scribing in the presence of a coating nor does it discuss methods of preventing damage to the substrate while scribing. U.S. patent application Ser. No. 2006/0220183 SEMICONDUCTOR WAFER HAVING MULTIPLE SEMICONDUCTOR ELEMENTS AND METHOD FOR DICING THE SAME, inventors Makoto Asai, Muneo Tamura, Kazuhiko Sugiura and Tetsuo Fujii, published Oct. 5, 2006 describes a two step process for scribing wafers having two layers where the layers have different refractive indices with respect to the laser wavelength being used. This application discusses only frontside scribing rather than backside scribing and does not discuss applications where the layer to be processed does not have a refractive index, but is in fact opaque to the wavelengths in question. Further this application does not discuss ways to avoid damage to the singulated devices as a result of the laser energy applied.
There remains a continuing need for a laser scribing process that scribes the backside opto-electric device substrates with reflective coatings in an efficient fashion without damaging the substrate using ultrafast laser pulses.